Electrographic digital printers are known, see, for example, WO 98/39691 A1 (=U.S. Pat. No. 6,246,856 A). In such a printing device or copying machine, charge images of the images to be printed are produced on a charge image carrier, for example, a photoreceptor belt, by means of a character generator. Subsequently, the charge image carrier is moved past developer stations, one for each color. These developer stations transport developers consisting, for example, of a toner and a carrier, to the charge image carrier. In accordance with the charge images on the charge image carrier, the toner passes over to the charge image carrier and stains the charge images. In the next step, the toner images are reprinted onto a print substrate and fused with the print substrate. The precise method of the printing process is described in WO 98/39691 A1, the content of which is thus included in the disclosure.
Usually, thermosetting is used to fuse the toner images with the print substrate. For this purpose, for example, fusing rollers are used with at least one of them being heated. Also infrared radiators are used as a heat source. Thermosetting of the toner images on the print substrate involves that the print substrate still has a temperature of, for example, 120° C. or higher upon leaving the fuser station, which makes further processing of the print substrate difficult. In order to eliminate this disadvantage, it is known to cool the print substrate after it leaves the fuser station.
According to DE 42 35 667 C1 (=U.S. Pat. No. 5,557,388A), cooling air is blown on the print substrate in order to cool the print substrate. The cooling device used for this purpose comprises cooling surfaces equipped with openings. Via an air duct, cold air is supplied to the openings. The air flows from the openings under the print substrate where it forms a cooling air cushion. At the same time, air is blown onto the other surface of the print substrate, namely against the direction of travel of the print substrate.
Further cooling units are known, for example, from DE 38 38 021 C2 (=U.S. Pat. No. 4,959,693A), EP 0 758 766 B1 (=U.S. Pat. No. 5,805,969), DE 201 19 854 U1, U.S. Pat. Nos. 6,907,220 B2, 6,567,629 B2. There, for example, fans are used to cool a print substrate or rollers which are cooled from outside or inside.
Rollers can be produced, for example, from aluminum in an extrusion process. For this purpose, the rollers, for example, cooling rolls, should have a diameter of >250 mm, for example, 400 mm. If, during the production of such a roll in an extrusion process, the material is pressed through the die (pressing tool), and produces the pipe of the roll with this diameter, the pipe leaves the die in a warm condition and is not yet inherently stable to carry its own weight. Increasing the wall thickness, then, is disadvantageous for heat conduction, because a compact wall thickness constrains heat conduction. Moreover, it is not possible to produce thin fins, for example, cooling fins, since the fins cannot be additionally supported because of the closed shape of the die. However, the fins must be self-supporting and stable in order to accept the forces of the extrusion process. Therefore only thick and only a few fins are used. Since the heat transmission depends on the available surface of the pipe and the fins, the maximum heat flow volume to be transmitted is restricted in the production method described. This disadvantage results, for example, in the fact that in practice, several small rollers or water-cooled rollers are used to cool a print substrate after toner images have been fused. This results in increasing costs for cooling a print substrate, for example, at the start of a fuser station.